This is true of many gases, including the noble gases (neon, helium, etc.), as well as nitrogen, hydrogen, sulfur hexafluoride, and others.
Here’s a pool party where the organizers poured liquid N2 into the pool. The low temps created an entertaining fog above the water, but the cold N2 vapor blanketed the top of the pool and displaced the usual atmosphere, causing swimmers to pass out before they had any idea they were in danger:
Carbon dioxide can also asphyxiate, but is intensely irritating at elevated concentrations (think of the last time you beer-burped through your nose, except imagine a far stronger sensation, throughout your entire respiratory tract).
Carbon monoxide is acutely toxic and can kill at very low concentrations, even when there’s enough oxygen present to otherwise sustain life.
Having suffered from what I’m pretty sure, in hindsight, was low-level CO poisoning over several months, I can believe that. The issue isn’t that there isn’t enough oxygen, but that the blood takes up CO in preference to pure oxygen. My roommates and I simply felt tired all the time, no matter how much we rested - of course, that rest was done in the apartment that had the CO leak, so… I won’t go so far as to say we felt “fine” - obviously we felt tired - but we didn’t feel like we had any trouble breathing. Our problems happened after we put weatherstripping around the door and windows of the low-rent apartment we were sharing one January. We’re just lucky that even with that, the place leaked like a sieve.
That pool party video is rather sobering. You’d think that in an outdoor scenario the nitrogen wouldn’t be that much more concentrated - especially since nitrogen is lighter than oxygen - but I guess there was enough all at once, that people were overcome before it had time to dissipate.
It’s worse than that, because CO2 triggers the “I need to breathe” reflex. So asphyxiation with CO2 feels like being suffocated. I once observed someone kill a cage of hamsters using CO2, and it was awful.
CO is potentially dangerous to onlookers because even a small leak could cause them damage. But it’s painless. N2 is almost certainly the best choice, since it’s painless and also quite safe so long as enough oxygen is present – that is, a minor leak in the apparatus would not be risky to onlookers. It’s also abundant and cheap.
It’s a nightmare scenario. Bystanders want to jump in to help, but they risk being likewise overcome. Confined-space incidents are also a problem in industry, for the same sort of reason.
Expanding on this:
Many folks here have probably amused themselves by inhaling deeply from a helium balloon and then speaking with an altered voice. You probably felt fine afterwards, but if you were to inhale a second time from the helium balloon without an intervening breath of fresh air, there’s a good chance you’d fall over half-conscious. So sayeth the squeaky voice of experience.
